The present invention relates to petroleum recovery operations, and more particularly, to the use of pulse technology to enhance the effectiveness of waterflooding operations.
Where hydrocarbons reside within a subterranean reservoir, such hydrocarbons may be profitably extracted from the reservoir by a variety of recovery techniques. Conventional primary recovery techniques, e.g., recovering hydrocarbons which flow naturally to the surface because the reservoir pressure exceeds the surface pressure, typically succeed in recovering up to about 15% of the reserves in a hydrocarbon reservoir. Conventional secondary techniques, e.g., waterflooding, typically succeed in recovering about 20% to about 30% of the reserves.
Generally, the combination of a secondary recovery technique, e.g., waterflooding, with the use of pressure pulsing is thought to enable the recovery of up to about 30% to about 45% of the reserves. Pressure pulsing as referred to herein will be understood to mean deliberately varying the fluid pressure in the subterranean reservoir through the application of periodic increases, or “pulses,” in the pressure of a fluid being injected into the reservoir.
Existing methods of pressure pulsing are problematic for numerous reasons. Pressure pulsing has been performed through the insertion of a pulse-generating apparatus into a subterranean wellbore, often in a location at or near a set of perforations, wherein the apparatus generates a pressure pulse. This is problematic because it is difficult and expensive to perform routine maintenance on the apparatus; a workover rig is often necessary to remove the apparatus from its designated location within the wellbore, wait while the routine maintenance is performed, and then restore the apparatus to its previous location. This becomes even more costly when the wellbore is located offshore; for example, deepwater workover rigs cost $250,000 to $400,000 per day to operate. Still another disadvantage lies in the fact that a pulse-generating apparatus located within a subterranean wellbore can never be networked to pressure pulse multiple wells at one time; it can only pressure pulse the well in which it is located. Still another disadvantage lies in the fact that the power is typically provided by a pneumatic power source, which, inter alia, requires a large cylinder to generate a useful pressure amplitude, dampens the pressure wave, generally requires big exhaust valves, and is generally less reliable than certain other sources of power, e.g., hydraulic power sources.
Pressure pulsing has also been performed through the use of a pulse-generating apparatus attached to a wellhead located above the surface. Pulsing typically occurs either by raising and lowering a string of tubing located within the wellbore, or by employing a flutter valve assembly which periodically opens and closes to permit a fluid to be pumped into the wellbore. The former operation is problematic because, inter alia, the amplitude of the pressure wave is fixed by the weight of the tubing; it is highly difficult to customize the amplitude for operations in wellbores where a narrow difference exists between the normal reservoir pressure and the pressure which fractures the reservoir. The latter operation is problematic because, inter alia, the means of pumping is limited; the periodic closure of the flutter valve assembly forecloses the use of a positive displacement type pump. Furthermore, neither operation continually maintains positive pressure on the subterranean reservoir. Rather, each operation emits a pressure pulse which briefly elevates the reservoir pressure, after which the reservoir pressure is permitted to decline, potentially back to the original baseline pressure. The inability to maintain a constant positive pressure on the reservoir, inter alia, can impair hydrocarbon recovery from the reservoir, and the stresses generated by alternating surges of positive pressure with gradual declines to neutral pressure may also adversely impact the longevity of the surface equipment and possibly the reservoir.
Additionally, no known pressure pulsing technique has reported achieving a pressure pulse with amplitude above about 500 psi; this is problematic in situations where an amplitude above about 500 psi may be required in order for pressure pulsing to beneficially impact hydrocarbon recovery.